Communication systems typically include a plurality of stations connected to one another over a common communication channel. For example, in a cable television network, a head end is connected to a plurality of subscriber stations by way of a cable plant. The cable plant typically includes a trunk line, a plurality of feeder lines connected to the trunk line, and a plurality of drop lines connected to each of the feeder lines by way of corresponding taps. Also, because long trunk and feeder line runs are typical in a cable television network, the cable plant also includes a plurality of amplifiers or repeaters which are positioned at periodic intervals along the trunk line and the feeder lines in order to boost the cable signal.
The trunk line, feeder lines, and drop lines of the cable plant support downstream communication from the head end to the subscriber stations and upstream communication from the subscriber stations to the head end. The portion of the cable plant which supports upstream communication is usually referred to as the upstream cable plant. Similarly, the portion of the cable plant which supports downstream communication is usually referred to as the downstream cable plant.
Accordingly, when the head end communicates with a destination subscriber station, it transmits a message through the downstream cable plant. That is, when the head end communicates with a destination subscriber station, it transmits a message downstream over the trunk line, then over the feeder lines to a tap to which the destination subscriber station is connected, and then over a drop line from the tap to the destination subscriber station. On the other hand, when a subscriber station communicates with the head end, it transmits a message through the upstream cable plant. Thus, when a subscriber station communicates with the head end, it transmits a message upstream through its drop line to a tap, through the feeder line to which the tap is connected, and then through the trunk line to the head end.
In a cable television network, the head end and subscriber stations must share the resources of the cable in some manner. For example, downstream and upstream communication is typically allocated to two different frequency ranges. In a sub-split allocation, the downstream transmission is allocated to a frequency range between 54 MHz and 750 MHz and above, while the upstream transmission is allocated to a frequency range below 42 Mhz. In a mid-split allocation, downstream transmission is allocated to a frequency range of 162 MHz and above, while upstream transmission is allocated to a frequency range between 5 to 100 MHz. In a high-split allocation, downstream transmission is allocated to a frequency range of 234 MHz and above, while upstream transmissions are allocated to a frequency range between 5 MHz and 174 MHz.
Moreover, the subscriber stations must also share the resources of the cable in some manner. In a time division multiple access (TDMA) cable television network, the subscriber stations share the cable by transmitting data during uniquely assigned and non-overlapping time periods. In a frequency division multiple access (FDMA) cable television network, the subscriber stations share the cable by dividing up the available frequency bandwidth into numerous narrow frequency channels and by allocating to each subscriber station its own corresponding narrow frequency band.
In a code division multiple access (CDMA) cable television network, the communication channel is divided up in neither the frequency domain nor the time domain. Instead, the subscriber stations share the cable by multiplying their data messages by corresponding assigned code words (which may be referred to as code vectors) and then transmitting the result. In effect, this multiplication spreads the data out in the frequency domain allocated to the station. To receive this data, the head end must correlate the data as received with its own set of related code words in order to recover the original data. Therefore, a CDMA cable television network is not constrained by time or frequency as are TDMA and FDMA cable television networks.
Moreover, because of attenuation of the data signals resulting from the propagation of the data signals through long lengths of cable in cable television networks, the data signals are typically amplified by the aforementioned amplifiers or repeaters to a high level at the head end and at various points along the length of the cable. Then, at the point where each subscriber station is connected to the feeder cable, the signal power level must be attenuated so as to supply a consistent signal power level to the subscriber station. The amount of attenuation that is required is typically within the range of 10 to 30 dB. Larger attenuation is required if the subscriber station immediately follows the head end or an amplifier and, because of the loss on the cable, progressively smaller amounts of attenuation are required as the distance between the subscriber stations and the head end or amplifier increases. The power level supplied to the subscriber station is usually controlled within the range of zero to 10 dBmV.
A significant problem associated with the upstream cable plant is ingress. Ingress is the pickup of stray radio wave signals (defined here as ingress noise) by the cable plant. Amplification within the upstream cable plant causes ingress noise funneling. That is, the head end sees all of the ingress noise picked up over all of the cable plant. A successful upstream cable television network must deal with the ingress noise in order to achieve best performance.
Associated with the ingress problem is a problem of egress. Egress is the radiation of electromagnetic radiation from the cable plant. Because the cable plant is an efficient receiving antenna for ingress, it is likewise an efficient radiating antenna for any signals that are transmitted over the cable plant. Ingress and egress result principally from poor shielding in the drop portion of the cable plant (i.e., the drop lines) and within the subscriber station.
The present invention is directed to a synchronized CDMA (SCDMA) system which is relatively unconstrained along both the time and frequency axes, which deals with ingress in an efficient manner, and which also reduces the power spectral density of any egress radiation that may be caused by the upstream signal.